Ouvrez le menu déroulant surCliquez dans la barre latérale

/* define a fixed width for the entire menu */

.navigation {
  width: 300px;
}


/* reset our lists to remove bullet points and padding */

.mainmenu,
.submenu {
  list-style: none;
  padding: 0;
  margin: 0;
}


/* make ALL links (main and submenu) have padding and background color */

.mainmenu a,
.mainmenu label {
  display: block;
  background-color: #CCC;
  text-decoration: none;
  padding: 10px;
  color: #000;
}

.mainmenu input {
  display: none;
}


/* add hover behaviour */

.mainmenu a:hover {
  background-color: #C5C5C5;
}


/* when hovering over a .mainmenu item,
  display the submenu inside it.
  we're changing the submenu's max-height from 0 to 200px;
*/

.mainmenu :checked+.submenu {
  display: block;
  max-height: 200px;
}


/*
  we now overwrite the background-color for .submenu links only.
  CSS reads down the page, so code at the bottom will overwrite the code at the top.
*/

.submenu a {
  background-color: #ddd;
}


/* hover behaviour for links inside .submenu */

.submenu a:hover {
  background-color: #993;
}


/* this is the initial state of all submenus.
  we set it to max-height: 0, and hide the overflowed content.
*/

.submenu {
  overflow: hidden;
  max-height: 0;
  -webkit-transition: all 0.5s ease-out;
}

<nav class="navigation">
  <ul class="mainmenu">
    <li><a href="">Home</a></li>
    <li><a href="">About</a></li>
    <li><label for="products">Products</label><input type="checkbox" id="products">
      <ul class="submenu">
        <li><a href="">Tops</a></li>
        <li><a href="">Bottoms</a></li>
        <li><a href="">Footwear</a></li>
      </ul>
    </li>
    <li><a href="">Contact us</a></li>
  </ul>
</nav>